Prior art electrical boxes are made from sheet metal in which a sharply defined shearing force normal to the surface of a wall member of the box is applied in a restrained manner to form a knockout area. The knockout area formed in this manner is typically in the form of a perforated depression in the surface of the wall member. A user can apply a force to the perforated depression to break the depression away from the wall member to form a knockout opening.
Metal electrical boxes have certain drawbacks, e.g. rust and corrosion, and more weather-resistant materials, such as plastic, offer advantages over metal electrical boxes.
A number of knockout forming techniques have been developed to allow a user to remove a portion of the wall member, e.g., a plug, to reveal or form a knockout opening. For example, stress concentration tabs may be used to initially hold the plug in the knockout opening. To remove the plug, a user simply breaks the tabs. This is generally accomplished by punching the plug into the interior volume of the electrical box.
Knockouts in which the plug is held in place by stress concentration tabs have a number of drawbacks. For instance, the plug does not seal the interior volume of the electrical box against precipitation. Also, once the plug is removed, it cannot be reattached to the wall member to effectively close the knockout opening. Further, the tabs allowed the plug to be removed by applying a punching force from outside the electrical box to force the plug inwardly into the interior volume of the electrical box. This allows the plug to be removed without having access to the interior volume of the electrical box, which creates the potential for tampering with the interior of the box.